linux-kernel - Re: [PATCH 1/5] sched_show_task: fix unsafe usage of ->real

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Date:	Tue, 11 Nov 2014 11:39:49 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Roland McGrath <roland@...k.frob.com>,
	Sterling Alexander <stalexan@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] sched_show_task: fix unsafe usage of ->real_parent

On Mon, Nov 10, 2014 at 11:00:26PM +0100, Oleg Nesterov wrote:
> rcu_read_lock() can not protect p->real_parent if release_task(p) was
> already called, change sched_show_task() to check pis_alive() like
> other users do.
> 
> Note: we need some helpers to cleanup the code like this. And it seems
> that that the usage of cpu_curr(cpu) in dump_cpu_task() is not safe too.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> ---
>  kernel/sched/core.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 240157c..1ec75c9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4572,8 +4572,10 @@ void sched_show_task(struct task_struct *p)
>  #ifdef CONFIG_DEBUG_STACK_USAGE
>  	free = stack_not_used(p);
>  #endif
> +	ppid = 0;
>  	rcu_read_lock();
> -	ppid = task_pid_nr(rcu_dereference(p->real_parent));
> +	if (pid_alive(p))
> +		ppid = task_pid_nr(rcu_dereference(p->real_parent));
>  	rcu_read_unlock();
>  	printk(KERN_CONT "%5lu %5d %6d 0x%08lx\n", free,
>  		task_pid_nr(p), ppid,
> -- 
> 1.5.5.1
> 
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